In recent years there has been much interest in composite ceramic materials. This is because some composite materials possess properties which are significantly improved over the properties of the individual constituents. These ceramic composites, including both fiber-reinforced and multicomponent structures, have been targeted as applicable to a variety of scientific and technological uses. Some of these uses include tooling applications, indenters, nozzles, and so forth. For these and other uses the desirable material should be as lightweight and as tough as possible; however, the attainment of one of these properties has in many cases been accomplished at the expense of the other property.
Boron carbide has been found to exhibit excellent hardness and a relatively low specific gravity, but it lacks toughness (K.sub.Ic =3.6 MN/m.sup.3/2). Titanium diboride, on the other hand, is nearly as hard and much tougher when compared with boron carbide, but it is also much heavier. Because of the potentially complementary properties of these two materials, researchers have directed attention to producing composites comprising both compounds. Results of this research indicate that a ceramic produced therefrom approximates titanium diboride's toughness and exceeds boron carbide's hardness while maintaining a low specific gravity. For example, U.S. Pat. No. 2,613,154 discloses the manufacture of titanium diboride/boron carbide composites from a mixture of titanium powder and boron-rich boron carbide powder. This method does not, however, appear to be suitable for producing a variety of titanium diboride/boron carbide compositions without the incorporation of excess carbon or boron in the densified piece. The same problem is encountered in connection with research done by Russian workers, as disclosed in E.V. Marek, "Reaction of Boron Carbide with Group IV Transition Metals of the Periodic Table," Mater. Izdeliya, Poluchaemye Metodom Poroshk. Metall., Dokl. Nauchn. Konf. Aspir. Molodykh Issled. Inst. Probl. Materialoved. Akad. Nauk Ukr. SSR, 6th, 7th, Meeting Date 1972-1973, 156-9. This paper describes mixtures of boron, carbon and titanium which are hotpressed to composites comprising B.sub.4 C and TiB.sub.2 phases. A microhardness superior to that of either B.sub.4 C or the borides is reported.
Japanese Patent Application 1985-235764 discloses boron carbide/titanium diboride composites prepared by dispersing boron carbide powder and titanium diboride powder in organic solvents such as toluene, and ball milling using a tungsten carbide-cobalt alloy as a milling medium. This material is then dried and coldpressed. The authors report an extreme hardness for a sintered piece prepared from 40 to 50 percent titanium diboride.
U.S. Pat. No. 4,029,000 discloses a boron carbide/-titanium diboride composite, prepared from a physical mixture of boron carbide and titanium diboride powders, for use as an injection pump for molten metals. The particle diameter is in the range of 2 to 6 .mu.m for the boron carbide and 5 to 15 .mu.m for the titanium diboride. The hardness attained upon sintering is reported to be lower than that of boron carbide alone.
Research has also been directed toward other composites comprising titanium, boron and carbon. For example, the literature also describes various methods of preparing composite materials comprising titanium carbide and titanium borides. Among these are, e.g., U.S. Pat. Nos. 4,138,456 and 3,804,034, which describe preparation of a TiC/TiB.sub.2 composite and a TiC/TiB/B.sub.4 C composite, respectively, produced from physical mixtures of powders. U.S. Pat. No. 4,266,977 discloses preparation of a composite prepared in a plasma reactor from an "intimate" mixture of the three constituents.
An important factor in the ultimate utility of a ceramic composite is the degree to which the constituents are dispersed. To realize the maximum benefit of a particulate composite, the components should be uniformly distributed on a microscopic scale. However, such uniform distribution is at best extremely difficult to attain in physical mixtures, such as those produced using any of various milling techniques, in part because of agglomeration of component particles. Physical mixtures are defined as mixtures of components in which the starting and ending components are the same. This is in contrast with the in situ production of ending components by various means.
A further consideration in producing an "ideal" composite material relates to particle size. This is because the high incidence of failure in engineered ceramic parts can often be attributed to small cracks or voids which result from incomplete packing of the precursor powders. A solution to this problem would be to use extremely fine composite powders that are substantially uniform as to particle diameter. Such powders would pack more tightly and thereby reduce the number of void spaces formed in the ceramic body. It has been suggested, by E. A. Barringer and H. K. Bowen in "Formation, Facking and Sintering of Monodispersed TiO.sub.2 Powders," J. Amer. Ceram. Soc. 65, C-199 (1982 ), that an "ideal" ceramic powder for producing a high quality part would be of high purity and contain particles which are substantially spherical, nonagglomerated, and both fine and uniform in size.
As a ceramic powder is sintered, adjacent particles fuse into grains. In general, the grain size is governed by the particle size of the powder from which the part is prepared. Thus, the sintering of finer particles presents the opportunity to produce fine-grained bodies. This is especially important in TiB.sub.2 /B.sub.4 C composites, in which the TiB.sub.2 and B.sub.4 C grain sizes are preferably less than or equal to about 20 microns in order to maximize the hardness and toughness of the composite. Thus, the particle sizes should preferably be significantly smaller than 20 microns. The effect of grain size on the integrity of boron carbide bodies having no titanium diboride constituent has been investigated by A. D. Osipov et al., "Effect of Porosity and Grain Size on the Mechanical Properties of Hot-Pressed Boron Carbide," Sov. Powder Metall. Met. Ceram. (Engl. Transl.) 21(1), 55-8 (1982). The authors disclose that parts exhibiting a fine grain size are stronger than parts consisting of coarse grains.
In order to produce the desired particle sizes, especially in the under-20 micron range, it is often necessary to mill the powder. While milling does promote size reduction, it is time consuming, may impart impurities such as metals, and does not decrease size beyond a certain point, even with additional milling time. This point depends on both the substance being milled and the chosen milling technique. See, e.g., W. Summers, "Broad scope particle size reduction by means of vibratory grinding, Ceramic Bulletin 62(2) (1983) 212-215. For most techniques boron carbide, a hard substance, reaches its minimum size at around 1 micron in diameter using attrition milling techniques. Further steps, such as acid leach and washing steps, may be required subsequent to milling in order to increase purity of the product.
Thus, what is needed is a method of producing a composite titanium diboride/boron carbide ceramic powder that is fine, uniform and of high purity, and which shows a high degree of mixing of the constituents. The method should reduce or eliminate the need for extended milling or purification procedures subsequent to production.